In portable radio devices, particularly in mobile stations we prefer to avoid the use of an antenna for convenience, which projects outside the cover of the device. In most cases internal antennas of mobile stations have a planar structure: The antenna comprises a radiating plane and a ground plane in parallel with it. In order to facilitate the impedance matching the radiating plane and the ground plane are usually interconnected at a suitable point by a shorting conductor, whereupon a planar inverted F-antenna (PIFA) is produced. The electrical characteristics of the planar antenna, such as the bandwidth and the antenna gain, depend on the distance between said planes, among other things. As the mobile stations become smaller also in the direction of the thickness, said distance is reduced unavoidably, whereby the electrical characteristics become poorer. This problem relates particularly to foldable mobile phones, as their fold parts are relatively flat. In practice such foldable models have projecting antennas.
The space utilisation of a radio device can be improved i.a. by arranging the radiating element of the antenna as a part of the device cover, which is known as such. The applicant knows the arrangement described in his own application FI20030059, where the radiating cover element has electromagnetic feed in order to obtain further advantages. FIGS. 1a and 1b show a solution of this kind. FIG. 1a shows a magnified cross-section of the antenna 100. There is a part 130 of the cover of the radio device, which functions as the radiator and below it the ground plane 110 of the antenna. A thin dielectric layer 105 lies against the slightly curved internal surface of the radiator 130 and a strip-like feed element 120 of the antenna lies on the surface of the dielectric layer. The layer 105 and the feed element 120 can together form for instance a flexible circuit board. Between the radiator and the feed element there is only an electromagnetic coupling, which is considerably strong due to the thinness of the dielectric layer. The antenna's feed conductor 116 and the shorting conductor 115 are galvanically connected to the feed element 120. The feed conductor extends through the ground plane to the antenna port of the radio device, insulated from the ground plane. The shorting conductor connects the feed element directly to the ground plane at the short circuit point S.
FIG. 1b shows the antenna 100 from outside of the device. There the radiator 130 is for instance one half of the mobile phone's back cover. The feed element 120 is represented by a broken line. In this example it is a conductor strip in a form resembling a T-letter, the stem of which extends in the width direction of the radio device, across the radiator, and the perpendicular “crossbeam” extends in the length direction of the radio device, close to one side edge of the radiator. The antenna's feed point F and the short-circuit point S mentioned above are located about in the middle of the stem. The short circuit point divides the feed element into two parts so that the antenna has two operating bands. The first part 121 of the feed element together with the radiator and the ground plane resonates in the range of the antenna's lower operating band, and the other part 122 of the feed element together with the radiator and the ground plane resonates in the range of the antenna's upper operating band. Thus the lengths of the first and second parts do not as such correspond to the wavelengths at the operating bands, but the coupling to the relatively large radiating element increases the electrical lengths of the parts of the feed element, so that these correspond to the intended wavelengths. It is also possible to excite such resonances in the antenna structure 100 which mainly depend only on the size of the radiator and on its distance from the ground plane. A resonance of this kind can be arranged for instance in the range of the upper operating band in order to widen it. For this purpose FIG. 1b shows a tuning element 140 drawn by broken line, which element is a conductor strip close to the feed element 120, and it is separated from the radiator 130 in the same manner as the feed element. The tuning element 140 is galvanically connected to the ground plane. FIG. 1b shows this connection, as well as the ground connection of the short-circuit point S, by a graphic symbol.
The antenna structure described above provides considerably broad bandwidths even in a flat radio device beside the fact that the radiator does no occupy space within the device also because the distance between the ground plane and the feed element, due to the relatively wide radiator, can be made slightly shorter than the distance between the ground plane and the radiating plane in a corresponding PIFA. However, improvements in the electric characteristics of the antenna are always desirable in order to secure the quality of radio connections.